Strain rate reduction tests are made on high purity iron containing of 14ppm C, S20C and S45C at very high strain rates. In order to evaluate quantitatively the strain rate dependence of the dynamic flow stress at each strain rate, decrement strain rate tests are performed at strain of about 0.20 at the strain rates above about 1 × 10^4/sec. The reduction ratios caused by the collision between an impact bar and decelerators are 0.44 and 0.586 of the strain rate before reduction. Activation volumes are determined from the amounts of the flow stress drop obtained from the reduction tests and the relation of a thermal activation process. The magnitude of activation volumes determined for the given materials depends not only on the strain rate but also on the difference in the carbon contents.
